System and method for manufacturing embedded conformal electronics

ABSTRACT

A method for fabricating an electronic device comprises providing a substrate ( 501 ), direct writing a functional material by a thermal spray on the substrate ( 502 ) and removing a portion of the function material to form the electronic or sensory device ( 503 ).

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of grant no.N000140010654, awarded by the Department of Defense, DARPA.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to conformal electronic devices, and moreparticularly to a method of fabricating conformal electronics usingadditive-subtractive techniques.

2. Discussion of the Prior Art

The adoption of computer-based design, engineering, and analysis toolsover the past 10-20 years has resulted in a tremendous acceleration inthe development cycle of modern engineering systems. Modern engineeringsystems are lighter, smaller, last longer, are more efficient, and arefar more reliable than their predecessors of even a few years ago. As aconsequence, however, these very same engineering systems are becomingextremely complex, with the result that the costs involved to repairsuch systems, particularly for major component failures, areskyrocketing. Accordingly, the ability to monitor the health of vitalengineering components in-situ and non-invasively in real-time is avital capability that is needed for modern engineering system designs tobe fully utilized, so that maintenance costs can be minimized, systemhealth monitored, and major repairs scheduled for the most opportunetimes.

The sensor system should not disturb or alter any aspect of the systemit is interrogating. However, after-market sensors, even if attachedduring the manufacturing process, can be unreliable, difficult toinstall, and may adversely affect component operation.

Electronic manufacturing with feature sizes in the meso-scale regime(e.g., about 10 to 1000 micrometers) often needs multi-step processesthat include time-consuming photolithographic methodologies. The timeneeded between iterations can often be measured in terms of weeks. Inaddition, thick film electronics based on ceramic multi-chip moduletechnology, including low temperature co-fired ceramic modules (LTCC-M)and high temperature co-fired ceramic modules (HTCC-M) generally needfiring of screen printed pastes to moderate ˜800 C. for LTCC-M or high1400 C for HTCC-M. The high temperature curing process gives rise toissues associated with mismatch in thermal expansion between dissimilarmaterials and can lead to premature debonding. This needs to beaccounted for during the processing through careful tailoring of theproperties of the layered materials. Current screen printing technologyis inherently limited in its fine feature capabilities, with the linewidth being limited to 100 microns or higher.

Therefore, a need exists for a system and method of fabricatingconformal electronics using additive-subtractive techniques.

SUMMARY OF THE INVENTION

Thermal spray technology coupled with precision laser materialsprocessing has been developed for the fabrication of electronics andsensor fabrication. Thermal spray is implemented for depositing a widevariety of materials that have functional properties as deposited. Thematerials generally do not need subsequent post-firing, annealing, orother time consuming, costly post processing steps. A variety ofmaterials can be deposited quickly and easily using thermal spraytechnology. After the deposition, precision laser micromachining using,for example, ultrafast or UV laser systems, can be used to fabricatecomplex electronic structures. The electronic structures include, forexample, resistors, capacitors, coils, transformers, and a variety ofsensors, for example, thermistors, thermocouples, thermopiles, strainsensors, magnetic sensors, humidity sensors, gas sensors, flow sensors,heat flux sensors, etc. Furthermore, these sensors can be embeddedwithin a component during manufacture to provide an extremely robust,long-life sensing and health monitoring system for the component, whichis superior to aftermarket, add-on sensors that must be attachedmanually using adhesives or other post-manufacturing techniques. Also,because the thermal spray technique is self-compatible, it can be usedto fabricate three-dimension electronics and sensor systems, e.g.,multi-layer sensors on the same surface area footprint, multiple-layerthermopiles for enhanced power production, etc.

A method for fabricating an electronic device, comprises providing asubstrate, depositing a functional material by a thermal spray on thesubstrate, and removing a portion of the functional material to form theelectronic or sensory device.

The substrate is flexible. Depositing is a direct writing.

Depositing a functional material further comprises heat treating thefunctional material. The heat treating is preformed one of before orafter removing a portion of the functional material.

Depositing further comprises forming a conformal layer on the substrate.Depositing the functional material further comprises providing one of ametal, a semiconductor, a ceramic, and a polymer in the thermal spray.Depositing the functional material further comprises providing one of adielectric material and an insulating material.

Removing the portion of the functional material further comprisesproviding a focused laser beam to the functional material.

The electronic device is fabricated in-situ.

The method comprises coating a portion of the electronic device.

The method further comprises depositing an insulating layer over thefunctional material after removing the portion, wherein the functionalmaterial is a bottom metal comprising at least two parallel strips,wherein a portion of each of the two parallel strips is exposed on eachof at least two sides of the insulating layer, depositing a top metal offunctional material by the thermal spray over the insulating layer andexposed portions of the two parallel strips, and removing a portion ofthe top metal of functional material, forming at least one strip, the atleast one strip connecting a portion of one of the two parallel stripsexposed on a first side of the insulating layer and a portion of asecond strip of the two parallel strips exposed on a second side of theinsulting layer.

A system for fabricating an electronic device comprises a thermal spraydevice for depositing a conformal layer of a functional material, and amaterial removal device for fabricating an electronic device from theconformal layer of the functional material.

The system comprises a fixture for retaining a substrate upon which theconformal layer of the functional material is deposited.

The material removal device comprises a programmable motion device. Theprogrammable motion device comprises a processor for receivinginstructions and an articulated arm supporting the material removaldevice proximate to the conformal layer of the functional material, thearticulated arm following the instructions received by the processor.The programmable motion device comprises a processor for receivinginstructions and an articulated stage supporting the conformal layer ofthe functional material proximate to the material removal device, thearticulated arm following the instructions received by the processor.

The material removal device comprises a laser. The material removaldevice is one of a water jet, a mechanical milling machine, andelectronic discharge machine.

The functional material is functional as deposited.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will be described belowin more detail, with reference to the accompanying drawings:

FIG. 1 is a Ni—Cu strain gauge deposited by thermal spray, forming aK-type thermocouple, exemplifying the use of a selective overcoat,according to an embodiment of the present invention;

FIG. 2 is a diagram of laser processing of thermal spray deposit tofabricate a strain gauge according to an embodiment of the presentinvention;

FIG. 3 is a diagram of a laser patterned Ni—Cu strain gauge deposited bythermal spray according to an embodiment of the present invention;

FIG. 4 is a graph of resistance versus strain for a NiCr thermal spraystrain gauge patterned using a laser system;

FIG. 5 is a diagram of the stages of thermal spray/laser patterning of amultiplayer thermopile according to an embodiment of the presentinvention;

FIG. 6 is a diagram of a multilayer thermopile, showing connectivitybetween bottom and top layers according to an embodiment of the presentinvention;

FIG. 7 is a diagram of a 40-element thermopile fabricated with NiCr/NuCuon alumina, with both positive and negative connector leads on theleft-hand side of device according to an embodiment of the presentinvention;

FIG. 8 is a diagram of a “star” thermopile concept, where the twothermocouple materials are represented by different shaded lines,according to an embodiment of the present invention;

FIG. 9 is a close-up diagram of the star thermopile interface betweendissimilar materials at inner and outer radii according to an embodimentof the present invention;

FIG. 10 is a diagram of a micro-heater laser patterned into NiCr coatingon an alumina substrate according to an embodiment of the presentinvention;

FIG. 11 is a graph of heater temperature versus input power for heaterin FIG. 10;

FIG. 12 is a diagram of an ultrafast laser trimmed of thermal spray lineaccording to an embodiment of the present invention;

FIG. 13 is a diagram of laser-machined vias in a multilayer thermalspray structure according to an embodiment of the present invention; and

FIG. 14 is a diagram of a laser trimming process according to anembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Direct write electronics technologies provide an opportunity tointegrate mesoscopic electronic devices with the physical structure onwhich the electronic systems will be used, eliminating the need for atraditional printed circuit board. The ability to print electronicfeatures on flexible and conformal substrates enables uniqueapplications for deployable electronics, such as placing electronics inprojectiles, for flexible satellite solar arrays, usage in rolled-upforms that can be inserted into symmetric or odd shapes, installed onmilitary gear, as well as various surveillance equipment. This can savespace and reduce weight through 3-D integration. It can provide adramatic cost savings by eliminating the majority of passive componentsin automated fabrication, while minimizing procurement. It can reduceinventories of electronic components or parts, enable the building ofspecialty parts on the “fly” without mass production set-up costs, andincrease the reliability of rugged electronic components due to theautomated assembly process and the absence of solder joints.

According to an embodiment of the present invention, thermal spraytechnology coupled with precision laser materials processing have beendeveloped for the fabrication of electronics and sensor fabrication.Thermal spray is implemented for depositing a material having functionalproperties as deposited, e.g., without the need for subsequentpost-firing, annealing, or other time consuming, costly post processingsteps in most cases, although these processes can be performed ifdesired. A variety of materials can be deposited quickly and easilyusing thermal spray technology. After the deposition, precision lasermicromachining using, for example, ultrafast or UV laser systems, can beused to fabricate complex electronic structures, for example, resistors,capacitors, coils and transformers, and can also be used to fabricate avariety of sensors, for example, thermistors, thermocouples,thermopiles, strain sensors, magnetic sensors, humidity sensors, gassensors, flow sensors, heat flux sensors, etc. Furthermore, thesesensors can be embedded within a component during manufacture to providean extremely robust, long-life sensing and health monitoring system forthe component, which is superior to aftermarket add-on sensors that needto be attached manually using adhesives or other post-manufacturingtechniques. Also, because the thermal spray technique isself-compatible, it can be used to fabricate three-dimension electronicsand sensor systems, e.g., multi-layer sensors on the same surface areafootprint, multiple-layer thermopiles for enhanced power production,etc.

A sensor that is directly embedded into the component in a coordinatedmanner has substantial advantages in terms of reliability, longevity,and minimal disturbance of component function.

According to an embodiment of the present invention, a system and methodhas been developed for the fabrication of sensors and electronics forcondition based maintenance and remote health monitoring of engineeringsystems. Direct-writing technology can be implemented for wide rangingfunctional electronics and sensor structures including metals,semiconductors, ceramics and polymers on virtually any surface.Direct-write line widths can be in the range of 200 microns and larger.According to an embodiment of the present invention, single layer andmulti-layer electronic devices can be fabricated through additivemask-free, environmentally benign electronics processing technology.Direct writing systems can be used for prototyping concepts inmanufacturing as well as provide new capabilities for the fabrication ofnovel embedded electronics and sensor systems.

According to an embodiment of the present invention, systems and methodscan combine additive-subtractive fabrication using direct write thermalspray for material addition, followed by and ultra fast, UV, or otherlaser processing for material removal. This can allow a substantialreduction in line width to the 10-micron level and below, as well as theability to use virtually any material. This approach can enhance theflexibilities of both processes, e.g., flexibility of thermal spray todeposit virtually any material/create multiple layers on low temperaturesubstrates, and the advantage of ultra fast or UV pulsed lasers tonon-thermally remove materials with minimal thermal damage. Othermaterial removal systems can be used, for example, a water jet,electronic discharge machining, or milling machine.

The capabilities include demonstration of the hybridized thermalspray/laser subtraction concept for an embedded sensor system for remotehealth monitoring of harsh environment engineering systems. An extendedcapability will involve incorporating wireless concepts for passive orsemi-passive embedded sensors using R-L-C circuits for untetheredmonitoring of the components.

The potential applications of such technology are unique andfar-reaching. Examples include strain gauges, thermistors,thermocouples, thermopiles (thermocouples in series for powergeneration), magnetic and piezo sensors, interdigitated capacitors forL-C circuits, antennas, microheaters (for integration into chemical andbiological sensors), among others. It will allow novel sensor andelectronic devices to be prepared in-situ and, to do so inenvironmentally friendly lean manufacturing methods.

Direct write electronics technologies provide an opportunity tointegrate mesoscopic electronic devices with the physical structure onwhich the electronic systems will be used, eliminating the need for atraditional printed circuit board. The ability to print electronicfeatures on flexible substrates enables unique applications fordeployable electronics, such as placing electronics in projectiles, forflexible satellite solar arrays, usage in rolled-up forms that can beinserted into symmetric or odd shapes, installed on military gear, aswell as various surveillance equipment. This saves space and reducesweight through 3-D integration. It provides a dramatic cost savings byeliminating the majority of passive components in automated fabricationand minimizing procurement. It reduces inventories of electroniccomponents or parts, enables the building of specialty parts on the“fly” without mass production set-up costs, and increases thereliability of rugged electronic components due to the automatedassembly process and the absence of solder joints.

According to an embodiment of the present invention, a system and methodcombines the thermal spray capabilities with complementary precisionlaser subtraction to provide substantially improved capabilities formanufacturing embedded, conformal electronics and sensors. For example,according to an embodiment of the present invention, the materialversatility of thermal spray for material deposition and coursepatterning is coupled with the fast, precision material removalcapabilities of ultra fast and UV lasers, which use non-thermal materialremoval mechanisms that minimize thermal damage associated with moretraditional laser processing. This combination capitalizes on thestrengths of both techniques: wide material versatility coupled withhigh-precision (˜10 μm) rapid patterning ability. Also, multi-layerstructures can be built with this technology, and electricalconnections, e.g., vias, have been successfully created to makeelectrical connections across both layers.

Thermal spray is a directed spray process in which material isaccelerated to high velocities and impinged upon a substrate, where adense and strongly adhered deposit is rapidly built. Material isinjected in the form of a powder, wire, or rod into a high velocitycombustion or thermal plasma flame, or wire arc, or a cold-spray(non-thermal) spray process, which imparts thermal and kinetic energy tothe particles. By controlling the plume characteristics and materialstate (e.g., molten, softened), it is possible to deposit a wide rangeof materials (metals, ceramics, polymers and combinations thereof) ontovirtually any substrate in various conformal shapes. The ability tomelt, soften, impinge, rapidly solidify, and consolidate introduces thepossibility of the synthesizing useful deposits at or near ambienttemperature. The deposit is built-up by successive impingement ofdroplets, which yield flattened, solidified platelets, and referred toas ‘splats’. The deposit microstructure and, thus, properties, asidefrom being dependent on the spray material, rely on the processingparameters, which are numerous and complex.

Thermal spray has been used for decades for large-scale applications,including, for example, TBCs in turbine engines, internal combustionengine pistons and cylinder bores, and corrosion protection coatings onships and bridges. Thermal spray can be used for meso-scale (e.g., about100 μm-10 mm) structures, particularly for electronic applications.Thermal spray methods can be used to form thick (e.g., greater thanabout 20 μm), smooth deposits of a wide range of ceramics, includingalumina, spinel, zirconia, and barium titanate. Additionally, thin(e.g., less than about 200 μm wide) metallic lines of Ag, Cu, as well asNi-based alloys, can be produced with square sides and that haveelectrical conductivities as good as, and in some cases superior to,conductor lines formed using thin-film methods. Spray productiontechnologies for coatings and direct-write lines include for example,combustion, wire arc, thermal plasmas and even cold spray solid-statedeposition.

The advantages of direct-write thermal spray for sensor fabricationinclude, for example, robust sensors integrated directly into coatings,thus providing unparalleled coating performance monitoring,high-throughput manufacturing and high-speed direct-write capability,and useful materials electrical and mechanical properties in theas-deposited state. In some cases, the properties can be furtherenhanced by appropriate post-spray thermal treatment. Further advantagesinclude being cost effective, efficient, and able to process invirtually any environment, robotics-capable for difficult-to-access andsevere environments, can be applied on a wide range of substrates andconformal shapes, and is rapidly translatable development tomanufacturing.

Thermal spray methods offer means to produce blanket deposits of filmsand coatings as well as the ability to produce patches, lines, and vias.Multiple layers can be produced on plastic, metal, and ceramicsubstrates, both planar and conformal. Embedded functional electronicsor sensors can be over coated with protective coating, allowingapplications in harsh environments. Such embedded harsh environmentsensors can be used for condition-based maintenance of engineeringcomponents.

High-power ultra fast laser systems, in which the laser pulse durationis measured in femto- or pico-seconds have advantages over theirthermal-based counterparts, including, minimal temperature rise andthermal damage in processed material, a wide range of applicablematerials, precision machining capabilities, sub-surface (3-D)machining, and high-aspect-ratio processing.

Ultra fast systems can use titanium-doped sapphire (Ti:sapphire) as thelasing medium, and chirped-pulse amplification (CPA) to producefemtosecond laser pulses with millijoule energy levels.

UV-wavelength, nanosecond-pulse lasers implement a pulse duration tensof thousands of times longer than an amplified femto-second system, anduse a wavelength in the UV region (typically about 355 nm or shorter),which results in direct bond-breaking by the incident photons. As such,like the ultra fast lasers, material is removed in a non-thermalmechanism (thermal damage is minimized), though not to the same extentas ultra fast lasers.

The use of ultra fast and UV lasers for precision materials processingworks well with a wide variety of thermal spray materials that can bedeposited for sensor and electronic applications. The combination ofthese two technologies provides for the capability to fabricate robust,embedded sensors in functional components.

Sensors can include, for example, thermistors and thermocouples fortemperature measurement as well as serpentine strain gauges for strainmeasurement. Temperature and strain are two of the most importantparameters in engineering systems such as internal combustion andturbine engines, power transmission systems, fluid power components,transportation equipment, general manufacturing systems, etc. Athermopile can be fabricated, which is a series of thermocouples (asmany as 100-200) in series to produce useful voltage and current, forpower generation in-situ using an existing temperature difference in thesystem.

The flexibility of thermal spray in its material deposition capabilitycombined with the simplicity and reliability of the thermocouple as atemperature sensor and the strain gauge as a strain sensor makethermal-spray-based thermocouples and strain gauges a natural choice.E-type and K-type thermocouples, and serpentine strain gauges can befabricated using variations of thermal spray. Substrates sprayed includepure alumina and spinel coated steel.

FIG. 1 shows a bare thermal-sprayed thermocouple (left) as well as athermocouple that has been coated with alumina (right) to demonstratethe ability to embed such sensors underneath functional coatings.

According to an embodiment of the present invention, thermal barriercoatings can be used to introduce a temperature difference in thepresence of an otherwise uniform heat load or temperature field. Thermalbarrier coatings (TBCs) can be been traditionally used to provideenhanced component lifetime in high temperature, harsh environments byproviding additional thermal resistance to heat flow to the device. TheTBC is thermal sprayed over the component, though other coatings forwear and corrosion can also be used. The TBC material is chosen to havea low thermal conductivity, hence in service heat will experience aresistance in moving from the top of the TBC to the component that isbeing protected underneath. Temperature differences of about 100° C. arecan be experienced. By using a TBC to selectively coat one side of athermopile, for example, a non-uniform temperature distribution would beproduced in the presence of a uniform heat flux, for example from aflame, or a panel exposed to solar radiation. The temperaturedifference, in turn, can be used with the thermopile concept to produceuseable electricity as discussed above.

Thermal spray technology can be used to fabricate integral strain gaugesdirectly onto system components or surfaces. Furthermore, thecombination of a strain gauge and a temperature sensor, which providesboth material temperature and compensation for the strain gauge,represent an extremely powerful combination. One popular material forhigh-temperature metallic strain gauges is NiCr. NiCr can be sprayed toform, for example, heaters and other laser patterned devices. Theinitial strain gauge development was based on NiCr, an inexpensive,readily obtainable material that also has useful properties.

Strain gauge fabrication using thermal spray can be obtained using asingle material for the sensor device itself. The same material, e.g.,NiCr, may also be used for both the strain sensor and the lead wires,provided the width and thickness of the lead wires are increased suchthat the effective resistance of the lead wire is negligible compared tothe strain gauge element. In practice this can be done by increasing thespray line width, while also depositing the lead wires at a slowervelocity—with the same material feed rate—to increase line thickness. Afive-fold increase in line width and thickness over the strain sensorline dimension, for example, results in a lead wire resistance of only4% that for an equivalent length of sensor patterning.

Referring to FIG. 2, the strain gauge pattern can be fabricated usingeither the ultra fast or UV lasers, and the patterns follow conventionalstrain gauge design, with a serpentine series of thermal spray tracesforming the gauge. Specific dimensions are determined by the desiredgauge resistance, size, sensitivity, and maximum expected strain.

Testing can be performed using precision multimeters or a standardWhetstone-bridge-based system to record resistance while the testspecimen is strained a known amount. During testing, a thermocouple canbe attached directly over the strain gauge to compensate for temperatureduring the measurement. A functioning prototype strain gauge fabricatedusing the ultra fast laser is shown in FIG. 3.

The results for a similar strain gauge fabricated using thermal spraytechnology followed by ultrafast laser materials processing is shown inFIG. 4. Repeatability between devices, in this case two gauges,linearity, and lack of hystersis are attributes of devices fabricatedaccording to an embodiment of the present invention. The gauge isfabricated on an alumina substrate, which is then fixed at one end as acantilever beam, while the free end is displaced a known amount. Acommercial strain gauge was attached to the sample as well to provide areference for the true strain of the specimen.

More sophisticated patterns are also possible, including depositing twomutually orthogonal patterns to measure strain in the x and y-directionssimultaneously. Thermal spray protective overcoats can be applied forprotection to the same gauge, which will then be re-tested to assess howthe overcoat influences gauge operation. It is also possible tofabricate arrays of strain sensors to determine variation in strain as afunction of location on a component. Strain gauge design can also bedesigned to minimize temperature drift.

Strain gauges are ubiquitous and indispensable in devices that rangefrom micro-weight scales to structure health monitors in buildings andbridges. Commercial devices are usually pre-fabricated, packaged andbonded or otherwise attached to the structure to be monitored. Ourapproach to mesoscale manufacturing allows strain gauges to befabricated in situ. Further, the sensor might even be hardened with afinal spray coat of a suitable impervious material.

In many remote sensor-monitoring situations, wireless concepts arerequired since access is not easy. For active wireless systems, localpower is essential to drive the circuit. One way to obtain this power,for example, in hot component monitoring, is power harvesting throughthermo-piles which is an extension to thermocouple technology.

Thermocouples produce a voltage proportional to the temperaturedifference across their junctions. As temperature sensors, they workvery well. Their output voltage, however, is on the order of severaltens of millivolts per ° C., making useful voltage levels for poweringelectronic circuits, e.g., 1-5V difficult without extremely largetemperature variations. A thermopile is a collection of thermocoupleswired electrically in series and thermally in parallel so that theirvoltages add. The idea is to fabricate a thermopile into a componentthat normally experiences some form of a temperature gradient duringoperation, e.g., an exhaust manifold, heat sink, friction-heatedsurface, or substrate for a chemical reaction. In the presence of atemperature difference, the thermopile will convert some of the heatflow directly to electric power, which can be used local activation ofcircuits.

The total thermopile output voltage (assuming a very high loadresistance so that current draw does not alter the voltage) isNS_(ab)ΔT, where N is the number of thermocouples, S_(ab) the Seebeckcoefficient, and ΔT the temperature difference between hot and coldtemperature sources. For a given thermocouple material and temperaturedifference, only N can be increased to increase the output voltage.Recent work has focused on the design and fabrication of multi-elementthermopiles for power generation and enhanced sensor applications usingthermal spray and MICE technology. A unique feature of this design isthe multilayer capability of thermal spray.

In this design a substrate is coated with an optional insulating layerand then the first alloy of the thermocouple (NiCr in this example) isdeposited. The sample is then sent to the ultra fast processinglaboratory in which the NiCr patch is cut into a collection of Nparallel strips. The sample is then sent back to the thermal sprayfacility for an insulating overcoat, followed by the deposition of thesecond thermocouple alloy (NiCr in this case). Finally the top layersare patterned using the ultra fast laser again to provide electricalseparation between layers, while providing an electrical seriesconnection. This is done by slightly staggering the top laser pattern toconnect the positive terminal of one thermocouple to the negative of thenext. Proof-of-concept designs were successfully completed with N=4. Fora K-type thermocouple (NiCr/NuCu), each thermocouple producedapproximately 5.5 mV for a total potential of ˜22 mV with a temperaturedifference of ˜125° C. A figure of the device in the various stages offabrication is shown in FIG. 5, and a schematic of the device is shownin FIG. 6.

Second-generation devices have been fabricated with N ranging from20-250. A recent K-type thermopile device with N=40 produced a voltageof ˜0.5V for a temperature difference of ˜300° C. between hot and coldjunctions, and is shown in FIG. 7.

In addition to the linear thermopile described above, a radialthermopile can also readily be fabricated, as shown in FIGS. 8 and 9. Inthis design, one junction is formed on the inside of the ring structure,and the second junction is formed at the outside ring. As for the linearthermopile the two thermoelectric materials are alternately depositedside by side and connected at their ends to form the thermoelectricjunctions. A heat source can be applied at the geometric center of thethermopile array, for example, by using a flame, torch, or by attachinga conducting material that is thermally connected to a heat source. Theouter edge of the circle is maintained at a lower temperature either bynatural means, for example, natural convection or by the use of fins, orby active cooling, using flowing gas, liquid or other means to maintaina temperature difference between the center and periphery of the starthermopile. Note that the device can work equally well by reversing theheat source and heat sink, e.g., by heating the edges and cooling thecenter.

Microheaters are resistive elements designed to deliver heat locally toa device. They find wide application in everything from gas flow sensorsto microfluidic lab-on-a-chip devices. A thermistor is a device whoseresistance is a sensitive (and known) function of temperature. Together,microheaters and thermistors allow closed-loop control of temperature,even under dynamic conditions such as ambient temperature or varyingthermal load. Suitable resistor materials can be deposited on a varietyof insulating subtracted included alumina and spinel, as well asplastic, wood, and ceramics. Similar to the strain gauge devicesdiscussed above, these materials are precision laser patterned using anultrafast or UV laser to form a heater element with the desiredgeometry, resistance, surface area, and temperature variation (ifdesired). Semiconductor thermistor material can also be deposited in thevicinity of the heater to operate as a thermistor sensing device. Such acombination will facilitate tighter temperature control and fasterresponse. Thermal sprayed thermistors as well as heater elements can befabricated. A photograph of the device is shown in FIG. 10, and thedevice temperature as a function of input power is shown in FIG. 11.

Thermal spray can be used to deposit thin lines of material fordirect-write of electronics. These lines, while achieving line widths of300 μm or larger, are difficult to fabricate in sized much smaller thanthis. Ultrafast laser processing can be used to pattern thermal spraydeposited lines for even finer feature resolution. To trim a thermalspray line, the laser makes multiple passes on both sides of the line,starting from the outside and working towards the center. The thicknessof the line is determined by stopping at a prescribed distance from thecenterline. The depth of the machining into the SPL and substrate isdetermined by the stage speed. The motion control system provides forpositioning accuracy of 0.5 μm.

An SEM image of a trimmed line is shown in FIG. 12. The material is Agsprayed onto a Ti substrate. The original line width as sprayed isroughly 500 μm. The laser-trimmed region is 80-100 μm in width, and 200μm in length. For this case, 10 strips were used on each side of theline with the laser making two passes over each strip. The stage speedwas 5 mm/s, and the process proceeds from the outside towards thecenterline of the line such that the final pass on each side is closestto the centerline, which is done to avoid re-deposition of material onthe trimmed portion of the line. Feature quality and uniformity aregood.

The laser-machined regions cut into the substrate as well as the SPL.This happens because there is no indicator at this time to instruct thelaser to stop cutting when the SPL line has been completely removed andthe substrate is being removed. To guarantee the entire SPL line wasremoved, the stage speed was run slower than needed. To optimize thetechnique, parameters can be empirically determined to providesufficient removal of material. Alternatively, the laser-processedregion can be dynamically monitored to determine when the substrate hasbeen reached. For example, the laser-processed feature can be monitoredusing a video camera or the ablated material can be analyzed using afiber-optic spectrometer, shutting off the laser when substrate materialbegins to be ablated.

Another observation made is that the trimmed lines are not perfectlysharp. Referring to FIG. 13, it can be seen that the spatial profile ofthe laser beam influences the trimmed line. The tighter the beam isfocused (for a smaller spot size), the more sharp the “hourglass” shapeof the beam becomes. If sharp, rectangular features are mandatory, itmay be possible to prescribe a more complicated laser-material path tominimize beam profile effects that tend to round the tops of the trimmedlines.

Vias can be fabricated into a thermal-sprayed multilayer structure usingthe motion control system. Feature quality can be improvedsubstantially. The vias, as with the handmade case, are done in athermal sprayed electrical inductor comprising several layers, forexample: Ti-substrate, bonding layer, ceramic insulator, bottom Agconductor, ceramic insulator, ferrous inductor material, insulator, andtop Ag conductor.

Feature quality and edge definition is very good. The perspective viewon the right in FIG. 14 is slightly deeper near the edges. This occursbecause the stage cannot accelerate or decelerate infinitely fast, andthe stage velocity is slower in this region, resulting in more pulsesper site and corresponding deeper features. This issue has beenaddressed and corrected recently.

Thermal spray technology is suited for developing multilayer sensors forenhanced performance. The thermopile concepts discussed above, forexample, can be extended by fabricated several devices on top of oneanother. For example subsequent linear thermopiles can be fabricated ontop of previous devices by thermal spraying an insulating layer betweendevices. In this fashion, all thermopiles would experience approximatelythe same temperature difference, however the individual devices could beelectrically connected in either parallel or series, depending on theneeds of the electrical load that the thermopile will drive.

Similarly, multiple sensors or devices could be fabricated on the samephysical area on a substrate, for example, a thermocouple fortemperature measurement, a strain gauge for strain measurement, amagnetic multilayer device and a microheater for periodic burn off ofcontaminants could be fabricated on the same physical footprint by usinga multilayer fabrication approach, and is a natural extension of thethermal spray capabilities and strengths.

Having described embodiments for a method of fabricating conformalelectronics using additive-subtractive techniques, it is noted thatmodifications and variations can be made by persons skilled in the artin light of the above teachings. It is therefore to be understood thatchanges may be made in the particular embodiments of the inventiondisclosed which are within the scope and spirit of the invention asdefined by the appended claims. Having thus described the invention withthe details and particularity required by the patent laws, what isclaimed and desired protected by Letters Patent is set forth in theappended claims.

1. A method for fabricating an electronic device, comprising: providinga substrate; depositing a functional material by a thermal spray on thesubstrate; and removing a portion of the functional material to form theelectronic or sensory device.
 2. The method of claim 1, wherein thesubstrate is flexible.
 3. The method of claim 1, wherein depositing is adirect writing.
 4. The method of claim 1, wherein depositing afunctional material further comprises heat treating the functionalmaterial.
 5. The method of claim 1, wherein the heat treating ispreformed one of before or after removing a portion of the functionalmaterial.
 6. The method of claim 1, wherein depositing further comprisesforming a conformal layer on the substrate.
 7. The method of claim 1,wherein depositing the functional material further comprises providingone of a metal, a semiconductor, a ceramic, and a polymer in the thermalspray.
 8. The method of claim 1, wherein depositing the functionalmaterial further comprises providing one of a dielectric material and aninsulating material.
 9. The method of claim 1, wherein removing theportion of the functional material further comprises providing a focusedlaser beam to the functional material.
 10. The method of claim 1,wherein the electronic device is fabricated in-situ.
 11. The method ofclaim 1, further comprising coating a portion of the electronic device.12. The method of claim 1, further comprising: depositing an insulatinglayer over the functional material after removing the portion, whereinthe functional material is a bottom metal comprising at least twoparallel strips, wherein a portion of each of the two parallel strips isexposed on each of at least two sides of the insulating layer;depositing a top metal of functional material by the thermal spray overthe insulating layer and exposed portions of the two parallel strips;and removing a portion of the top metal of functional material, formingat least one strip, the at least one strip connecting a portion of oneof the two parallel strips exposed on a first side of the insulatinglayer and a portion of a second strip of the two parallel strips exposedon a second side of the insulting layer.
 13. A system for fabricating anelectronic device comprising: a thermal spray device for depositing aconformal layer of a functional material; and a material removal devicefor fabricating an electronic device from the conformal layer of thefunctional material.
 14. The system of claim 13, further comprising afixture for retaining a substrate upon which the conformal layer of thefunctional material is deposited.
 15. The system of claim 13, whereinthe material removal device comprises a programmable motion device. 16.The system of claim 15, wherein the programmable motion devicecomprises: a processor for receiving instructions; and an articulatedarm supporting the material removal device proximate to the conformallayer of the functional material, the articulated arm following theinstructions received by the processor.
 17. The system of claim 15,wherein the programmable motion device comprises: a processor forreceiving instructions; and an articulated stage supporting theconformal layer of the functional material proximate to the materialremoval device, the articulated arm following the instructions receivedby the processor.
 18. The system of claim 13, wherein the materialremoval device comprises a laser.
 19. The system of claim 13, whereinthe material removal device is one of a water jet, a mechanical millingmachine, and an electronic discharge machine.
 20. The method of claim13, wherein the functional material is functional as deposited.